1. Field of the Invention
The invention relates to semiconductor wafer characterization equipment and, more particularly, to techniques for determining the number and location of defects such as contaminant particles that have been added or removed on a semiconductor wafer after semiconductor wafer handling or processing.
2. Description of the Background Art
Semiconductor wafer characterization typically involves a scanning process for locating defects on the surface of a semiconductor wafer. A scanning device, e.g., a laser scanner, scans the entire wafer surface to provide a general overview of the semiconductor surface and locates defects such as contaminant particles on the wafer surface. One such laser scanner device is the Tencor SurfScan 6200, manufactured by Tencor Instruments.
Specifically, the laser scanner device raster scans the wafer surface with a laser beam to locate defects on the wafer surface by analyzing laser backscatter from defects on the surface. The laser scanner device creates a laser scan map of the coordinates of the wafer features and contaminant particles. This laser scan map uses the coordinate system of the laser scanner device to identify the location of surface features and particles. The manner in which the laser beam is scattered from the wafer surface features and particles yields signals from which estimated defect locations in terms of x and y coordinates can be determined.
To characterize a wafer process, successive scans of the same wafer are typically taken during semiconductor wafer processing. For example, prior to processing, the scanning system, under computer control, scans and creates a first scan map of the entire surface of the semiconductor wafer. The initial scan generates a first data set of surface data of contaminant locations on the wafer surface at the time of the initial scan. Thereafter, the wafer undergoes a second scan after processing and/or wafer handling. A second scan map is created from the post-process scan. The second scan generates a second data set of surface data of contaminant locations on the wafer surface at the time of the second scan. Comparing the scans should identify pre-process particles from process-related particles; however, since the position of the wafer on the scanner platen during the second scan is invariably different from the wafer position used during the first scan, successive scans of the same wafer have different misaligned sets of data. Simply subtracting the second scan from the first scan results in particles from the first scan identified as process-related particles. Consequently, the scanner erroneously indicates process-related particles that are not process-related.
Therefore, a need exists in the art for an automated method for aligning wafer surface scans to facilitate identification of additional particle contaminant locations on a wafer surface after wafer handling or process operations.